Electronic Gurus, little help?

I just purchased these LEDs: http://dipmicro.com/store/HLMP-S500

for a project I'm doing for my car. I would like someone to double check my calculation on resistor sizing. I used 14.4V for supply voltage to be on the safer side. With the specs of the LED, I calculated each one should have a resistor of ~610 ohms.

Radio Shack has 680 ohmm 1/2 watt resistors: http://www.radioshack.com/product/in...odsInSession=1


Does this sound good? Think I can get away with a smaller resistor? I definitely don't want to risk burning these out, as it will be a PITA to replace them.

Thanks,

-Ian-

Looks like you're on the right path. Here's the math:

Your supply is 14.4V, the forward voltage drop of the LED is 2.2V. That leaves 12.2V to be dissipated across your resistor.

With a 610 ohm resistor, the current through your circuit is I=V/R, so 12.2V/610 = 20mA. That is well below the 30mA DC current spec'd for your LEDs, even if you account for resistor tolerances of about 5%, and maybe some supply fluctuation. So you're good on that front.

Now you have to make sure your resistor are sized well to deal with the power dissipation. Power = V^2/R, or 12.2*12.2/610 = 0.25 Watts. So your resistors can dissipate the power you're putting through them. 560 ohm resistors would be within your tolerances too.

At 470 Ohms, you'd be running 25mA through the LED, and 1/3 Watt through the resistor, which would be ok for the red or green LEDs but not the yellows. At 330 Ohms you'd be out of spec for all of them.

Where did you get the 30mA from? I see 20mA at the bottom of that page. That's what I used to figure out the 610 ohm resistor.

I'm not sure that Radio Shack carries a 560 ohm resistor... I would love to use the 470, as I want these as bright as possible, but I'm worried about exceeding the 20mA rating (unless you are going by the max rating).

The math is no problem, just haven't worked much with maximizing the output on raw LEDs, so I'm not familiar with how much you can push their limits.

radio shack's website lists a 560 ohm... or you can run a 470 and 100 ohm in series.... ~570 ohm.

If you want to run 500ohm.. you can also run 2x1000 ohm in parallel

Or a 220 and 330 in series.. ~550.

If you wire them in series.. just add them up.

If you wire them in parallel... R= 1/(1/R1+1/R2...)

Create your own combo to get as close as possible to the value you want.

Quote:

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Originally Posted by island boyeee

Where did you get the 30mA from? I see 20mA at the bottom of that page. That's what I used to figure out the 610 ohm resistor.

I'm not sure that Radio Shack carries a 560 ohm resistor... I would love to use the 470, as I want these as bright as possible, but I'm worried about exceeding the 20mA rating (unless you are going by the max rating).

The math is no problem, just haven't worked much with maximizing the output on raw LEDs, so I'm not familiar with how much you can push their limits.

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also.. read the tech specs..

http://www.dipmicro.com/?datasheet=HLMP-S500.pdf

30ma for all but yellow.. which is 20ma

I guess I should have prefaced this by stating I have my BSEE... so, all the math stuff is no issue. I'm also a car audio guru...

I see that the absolute max rating is 30mA but didn't think I should size the resistor to that. All of their tests were done at 20mA. Correct me if I am wrong.

These will be going in my headlights, so I'd like to stick to one resistor for ease of install and space requirements.

Quote:

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Originally Posted by island boyeee

I guess I should have prefaced this by stating I have my BSEE... so, all the math stuff is no issue. I'm also a car audio guru...

I see that the absolute max rating is 30mA but didn't think I should size the resistor to that. All of their tests were done at 20mA. Correct me if I am wrong.

These will be going in my headlights, so I'd like to stick to one resistor for ease of install and space requirements.

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Even their absolute max rating probably has some safety margin on it. I wouldn't hesitate going w/ honclfibr's suggestion of 470 for red/green.

20mA is probably safer then, especially seeing as they're going in your headlights, so the operating temps will be higher than normal.

So, you think the 560's will be a good compromise then? These will be Audi-style lights. I'm doing a HID retrofit, so yea there will be some heat in there.

Quote:

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Originally Posted by island boyeee

So, you think the 560's will be a good compromise then? These will be Audi-style lights. I'm doing a HID retrofit, so yea there will be some heat in there.

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I'd say so...

You can always try a circuit w/ 610 and see if the brightness is ok with you... and if not, then do the 560. You don't have to enclose it all... just wire it up freely.

680 was the other option, not 610. If Radio Shack has the 560's I'll go with that.

Thanks for the help and quick responses from all of you!

Get a few 904 nm LEDs and mix them in with the visible ones. LIDAR operates at 904 nm and the LEDs should easily washout the reflected LIDAR signal, making your car all but invisible to LIDAR, from the front.

uhhh.... how come i've never heard of this on any of the radar/lidar forums and threads i've read??

Quote:

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Originally Posted by island boyeee

uhhh.... how come i've never heard of this on any of the radar/lidar forums and threads i've read??

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Pure speculation on my part. Lidar operates by sending short pulses toward a target, measures the time it takes to reflect back, repeats this, and then compares the difference over time to calculate the speed then repeats and so on. If you send a continuous wave (at the same wavelength as the lidar transmitter) to the receiver on the Lidar, it can't see determine what the round trip delay is because there's no period where there is no light. It's the same as when you try to use an optical time domain reflectometer to measure a fiber optic cable and there's already light on the cable. It just can't be done. The only problem with this theory is if the lidar is so powerful or the reflector is so good, that the reflected light overpowers the LEDs. I imagine the closer you get to the Lidar, the more likely this would be, so you'd still have to slow down or risk a ticket.

haha nice theory there. let me know how that works out...